Confined spaces such as building interiors and packaging may have limited exchange of moisture vapor with adjacent or external environmental spaces. Materials having moisture permeance that is variable when exposed to different ambient conditions can provide benefits in such fields as construction, packaging, etc.
Buildings are commonly equipped with thermal insulation in order to provide comfortable living conditions inside the building. In addition to insulation, buildings are also constructed with various air barrier materials to limit air and water infiltration into the structure of the building. These air barriers minimize heat loss through draft flow.
In addition to air barriers, vapor barriers have also been used for controlling flow of moisture. For example, the moisture flow is generally from inside to outside when the exterior air is cold and dry (winter) and from outside to inside when the air is hot and humid (summer). In regions where these conditions prevail, it is common practice to use a vapor barrier such as polyethylene (PE) film on the warm side of the insulation, generally behind the gypsum wallboard, to prevent moisture diffusion from the interior heated space to the exterior cold environment. Such vapor barriers reduce the potential for moisture condensation in the interior wall cavity space as the water vapor is exposed to the cold exterior temperatures. Although the use of PE film as a vapor barrier reduces moisture condensation during exterior conditions of cold temperatures and low humidity, the same vapor barrier increases the likelihood of moisture condensation at the insulation-PE film interface during hot, humid summer conditions where moisture flow is from outside to inside. Therefore, it is desirable to have a material that acts as a vapor barrier during winter but facilitates vapor permeation during hot, humid summer conditions. It is also desirable to have a vapor barrier that can become vapor permeable under humid conditions in the wall cavity caused by unexpected uncontrolled events like water leaks into the wall cavity, to facilitate drying of the wall cavity by inward moisture flow as well as outward flow. Under conditions of low ambient humidity, low moisture permeance is desirable and under high ambient humidity, high moisture permeance is desirable.
U.S. Pat. Nos. 6,808,772; 6,878,455 and 6,890,666 disclose applications of a polyamide building liner material that has a water vapor diffusion resistance (WVDR) of from 2 to 5 meters diffusion-equivalent air space width at a relative humidity (RH) of an atmosphere surrounding the vapor barrier between 30% and 50% and a WVDR of less than 1 meter diffusion-equivalent air space width at a relative humidity between 60% and 80%. See also, US Patent Application Publication 2003/0215609 (a moisture-adaptive vapor-barrier film comprising PE and acrylic ester with a WVDR of from 0.5 to 100 meters diffusion-equivalent air space width at an RH of an atmosphere surrounding the vapor barrier between 60% and 80%) and WO2002/070351 (use of ionomers for sealing insulating materials having a WVDR of from 1 to 20 meters diffusion-equivalent air space width at an RH of an atmosphere surrounding the vapor barrier of 25% and a WVDR of from 0.02 to 0.7 meter diffusion-equivalent air space width at a relative humidity of 72.5%).
Variable vapor permeance can also be desirable of packaging. It is desirable for packaging of food items such as fresh produce to have low moisture permeance under conditions of typical cold storage (low temperatures and low humidity) to retain moisture within the package, thereby retarding desiccation of the produce and maintaining freshness. However, if the packaged produce is exposed to higher temperatures and humidity, such as in a warehouse that is not climate-controlled, increased respiration rates can lead to increased liquid water formation due to high levels of trapped moisture inside the package. Fresh fruits and vegetables also produce carbon dioxide during respiration and the carbon dioxide can react with moisture in the package to form carbonic acid. The carbonic acid formed can affect the quality of the produce. As a result, degradation of the produce by carbonic acid is accelerated. Packaging that allows moisture vapor to permeate at higher ambient humidity would allow for moisture equilibration with the exterior of the package, thereby helping to minimize formation of carbonic acid by reducing the available liquid water. As a result, the produce would be protected from degradation by carbonic acid, which could allow for extended storage of produce without loss of food quality, even under storage conditions of variable ambient temperatures.